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I think the point of errant limp home mode or truly a problem is lost on you. With an airplane engine you want power until it shells out. You want power to save you. You don’t care about the wreckage, you want to live. If the pilot sees reduced power will save him, his hand can reach up and throttle back. If full power for 2 minutes will save you, you blow it up. If you are flying over high terrain and weather keeps you from turning back and then your engine goes limp home, you choice to fly into the mountain is made for you. Full power for ten more minutes might be all you needed. You are not going to crack the code to turn it off. You probably will not even be able to toggle manual gearbox without a full chip burn. Minor programming tweaks like a chip just shift the data. It’s not whole new programming. When my Toyota truck goes limp home, it will not go 40 miles an hour. The limp home is hurting everything more than the emissions code that triggered it. That stuff can’t be an option on an airplane. If you could toggle it he mode that GM has where you can drain the coolant and drive 100 miles, that would be cool. But it would need to be in a switch not automatic.

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an automotive ECU is a poor choice as the programmed parameters are trying to keep the engine in the fuel efficient or emission compliant mode and they will constantly need a good O2 sensor signal (amongst all the others). In an aircraft application, the demand is different: full power (rich mixture) on take-off and climb-out and then lean for cruise if you care about the fuel burn. An automotive ECU might not give you full rich at the level and the time needed. In cruise, it will go into environment mode (lambda =1) so that the catalytic converter works. Now don't forget that the latest ECUs have 2 O2 sensors, 1 before and 1 after the catalytic converter. Both of those signals need to correspond to the fuel injection volume variations (the air-fuel ratio is not steady, it fluctuates ever so slightly either side of Lambda 1). If those signals don't correspond to each other, the yellow light will come on and it is anyones guess what action the ECU will take after some time.

Maybe go back to 2015 and read up on the VW diesel emission scandal, it provides some insight into the programming of these ECUs.

I would use any or all of the sensors but not an OEM ECU. They are not just a can of worms, they are a drum full of worms.

I don't think that the ecu will not be able to figure out how to make power. You open the throttle, the ECU knows how much fuel to add and fires the injectors.

Altering AFR may not be easy or possible, but we all know that car motors are just a big bag of trade offs. Will letting the ECU run mixture burn significantly more fuel than a manual mixture? I don't think so. Not SIGNIFICANTLY more. I think its more important to compensate for altitude first, then leaning for minimum fuel burn.

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I think the point of errant limp home mode or truly a problem is lost on you. With an airplane engine you want power until it shells out. You want power to save you. You don’t care about the wreckage, you want to live. If the pilot sees reduced power will save him, his hand can reach up and throttle back. If full power for 2 minutes will save you, you blow it up. If you are flying over high terrain and weather keeps you from turning back and then your engine goes limp home, you choice to fly into the mountain is made for you. Full power for ten more minutes might be all you needed. You are not going to crack the code to turn it off. You probably will not even be able to toggle manual gearbox without a full chip burn. Minor programming tweaks like a chip just shift the data. It’s not whole new programming. When my Toyota truck goes limp home, it will not go 40 miles an hour. The limp home is hurting everything more than the emissions code that triggered it. That stuff can’t be an option on an airplane. If you could toggle it he mode that GM has where you can drain the coolant and drive 100 miles, that would be cool. But it would need to be in a switch not automatic.

What ifs are just that. None of us really know what triggers limp mode, and if they can be mitigated via the ECU. I think that most of the triggers can be eliminated because the majority of limp home triggers are due to the transmission. Using a manual transmission configured ECU will remove all of those.

So lets make a guess, whats monitored on the engine that will cause limp home mode? Who's got a list? No one?

Now we have two schools of thought: limp home will kill you dead no matter what, even if they don't know what will trigger it, or limp home mode, if triggered, is safer than having parts come out the motor.

Noted. This thread is about fitting redundant spark and fuel to an engine with an OEM system. You don't think that's needed and you've said it several times. You think you can make an automobile OEM system work well in an airplane and no redundancy is warranted. Got it. Several folks with experience have disagreed with you. If you want to continue to push that, maybe start a new thread ("Why auto OEM EFI/EI systems work well in airplanes!") . You'll probably attract even more input from folks who know about auto OEM systems. Per the title, we're discussing how to fit redundant systems here.

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If we have the 555-based system (or something like it) to provide fuel (again, as a backup to the primary fuel system--EFI or carb), that still leaves us without a redundant ignition system if the OEM ECU crumps out (or which might only provide "limp-home" power that isn't sufficient to safely remain airborne, climb, etc).

For a redundant and totally independent ignition system (with possible exception of a single spark plug per cylinder), the simple "magnetron" system used by industrial engines, etc would seem to be hard to beat. TiPi provided a good post (with pictures) on that here. It would be pretty easy to set them up on any engine with a flywheel. The spark is fairly weak compared to a modern EI system, which could be a problem (best spark plug gap?) if we are sharing a sparkplug with a high-voltage primary system.
++++++++++
Industrial engines only: If we were starting with a 2 cyl industrial engine that already had a magnet on the flywheel and separate pickup/trigger coils for each cylinder, then a redundant ignition setup could, conceptually:
1) Use the same magnet (with the "secondary" coils perhaps staggered a bit to provide a different amount of advance) or
2) A second magnet could be mounted on the flywheel (180 degrees from the primary magnet?) and the new coils would be placed there.

In the real world, it doesn't look like there's enough real estate (at least on the 810cc engine) to use option 1.

It might also be possible to put the two systems at different timing points, as long as the engine would run "well enough" on either one.

On 2 cyl industrial engines, it's probably worth remembering (as TiPi pointed out in his post linked above) that the stock ignition systems are already independent for each cylinder. If one fails, the engine will keep running (though very poorly). If that's enough power to get your plane safely on the ground, then maybe more redundancy isn't worth the trouble.

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vigilant1:
Would a stronger, rare earth magnet give a hotter spark on a "magnetron" ignition? Is the Darlington transistor a relatively recent (<50 years) development? I seem to recall seeing small engines with points.

Well-Known Member

vigilant1:
Would a stronger, rare earth magnet give a hotter spark on a "magnetron" ignition? Is the Darlington transistor a relatively recent (<50 years) development? I seem to recall seeing small engines with points.

Reliability or performance. A hotter spark may need better insulation or cause parts to break down sooner or to heat up and momentarily lose power and then miraculously work just fine when they cool back down and a myriad of other problems.

Well-Known Member

vigilant1:
Would a stronger, rare earth magnet give a hotter spark on a "magnetron" ignition? Is the Darlington transistor a relatively recent (<50 years) development? I seem to recall seeing small engines with points.

the older style magnetos used the points to break the primary circuit. All small engine manufacturers have gone to an integrated solid-state switch that triggers on passing of one of the magnetic fields. It is not the most precise trigger but contained in one compact unit. Motor bikes generally use a separate trigger coil which gives a more precise timing. It also allows for ignition advance (retard at low rpm). Honda has that built-in the CDI coils on the GX630-690 series, retard is about 9deg below 1,000rpm (starting). Kohler did a similar retard on some V-twins but with an external module for both coils. Most of the others are fixed timing.

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Reliability or performance. A hotter spark may need better insulation or cause parts to break down sooner or to heat up and momentarily lose power and then miraculously work just fine when they cool back down and a myriad of other problems.

What is "hot?"
You need to differentiate between the stages of the actual spark: the initial voltage rise is critical for reliable sparks at the spark plug. Slow rise encourages fault path discharges eg under the spark plug cap, along the insulator and anywhere on the HT side where there is a lower resitance to ground. The spark can't jump across the gap until the air/gas between is ionised enough for that to happen. This is affected by the compression ratio, the spark plug gap, the size and type of surfaces either side of the gap and a few more.
The spark then has a number of different performance values:spark duration: how long will the secondary ignition system have enough voltage to maintain the spark acroos the gap? Turbulence in the combustion chamber can also blow-out a spark. Battery-powered systems have longer durations than CDI, CDI generally has a higher voltage risespark energy: how much energy is in the spark arc. This is the visible difference in sparks intensity for different systems. Higher energy has more power to ignite the mixture, add longer duration and that is the desired system.

The CDI on the B&S has quite a good spark voltage rise and a good voltage. I pulled about 16mm (~45,000V) from the ride-on mower engine at cranking speed. But the spark does look skinny, so not too much energy.

Stronger magnets don't necessarily result in stronger sparks, it all depends on when the saturation point in the primary coil is reached and the capacity of the capacitor. If the original magnet provides saturation, the only thing that might change is the timing.

Well-Known Member

Interesting. I didn't know about the slow rise problem. Hmmm... Not really my field, I admit. If I designed an ignition system it would probably use capacitive discharge from high voltage Leyden jars and would only work right on especially dry, even numbered days in Arizona. And that's if the SPCA didn't come after me for using all those cats to charge the Leyden jars.
I'd forgotten about the saturation issue. I guess beyond that point you don't get much extra zip from more current

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Of course, each "magnetron" generates a "wasted spark" during the exhaust stroke of that cylinder. It would be elegant if we could figure out a simple, reliable way to save that charge in the capacitor and then send the wasted spark to the other cylinder at the right time. No new magnets or coils needed. In practice, it would require too many bits and would probably wind up being less reliable than the stock setup. Also, IIRC, the normal wasted spark is fairly puny (unfavorable conditions at the spark plug?). It might stress the existing components more if they had to generate a very energetic spark with each revolution.

Well-Known Member

If the secondary voltages of the two ignition systems are similar, what is the effect of running them in parallel?
If the secondary ignition fires a few degrees later : will it damage the primary circuit?

That would be the purpose of the diodes mentioned in a previous post, to prevent the energy from one system from running back up the wire to the other system.
Diodes are reliable, but we would be introducing more components and that has a risk. Diodes can fail open (so no current flows in either direction) or closed (so they no longer serve as one-way valves, electricity can flow both ways).

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I'm aware of the diode approach: I was contemplating what the effect of feeding a "reverse" voltage into the first coil.
It will obviously induce a voltage in the coil's primary, reduced by the turns ratio.
Is that induced voltage going to damage anything, or will it be ok?

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I'm aware of the diode approach: I was contemplating what the effect of feeding a "reverse" voltage into the first coil.
It will obviously induce a voltage in the coil's primary, reduced by the turns ratio.
Is that induced voltage going to damage anything, or will it be ok?

I don't know for sure. Whatever energy does go to backfeed the other coil isn't going to the sparkplug, so that's not good. Also, I'm not sure how this reverse voltage will impact the Darlington transistor(s) in the trigger circuit, or the wires of the coils themselves. I guess somebody could test it.
The diodes prevent the situation from arising.

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Personally I am of the slap another engine on it and leave the stock ignition single spark plug system and single fuel system alone ilk. But if one was to slap two ignitions on a single plug per cylinder engine why not run four diodes for each plug? Maybe a special fitting between the coil and the high tension lead that houses four diodes, two diodes in series in each of two parallel paths. A simple way of checking that all four diodes work pre-flight is a bit of a head scratcher but some bright soul can figure it out. Diodes are small light and reasonably inexpensive. I guess you'd be four times as likely to have a diode fail but twice as safe that your ignition wouldn't back emf. The good thing is that with proper procedure a failed diode should be discovered prior to flight and shouldn't effect performance during a flight.